EXCHANGE 


ZIbe  IHntversitp  of  dbtcago  nr 


FORMATION  AND  LIFE  OF  METASTABLE 

HELIUM 


A  DISSERTATION 

SUBMITTED  TO  THE  FACULTY 

OF  THE  OGDEN   GRADUATE   SCHOOL  OF  SCIENCE 

IN  CANDIDACY   FOR  THE   DEGREE   OF 

DOCTOR  OF  PHILOSOPHY 

DEPARTMENT   OF  PHYSICS 


BY 

FABIAN  MILLER  KANNENSTINE 


Private  Edition,  Distributed  By 

THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 

CHICAGO,  ILLINOIS 


Reprinted  from 
THE  ASTROPHYSICAL  JOURNAL,  Vol.  LV,  No.  4,  May  1922 


ZTbe  Ulniverstts  of  Cbtcago 


FORMATION  AND  LIFE  OF  METASTABLE 

HELIUM 


A  DISSERTATION 

SUBMITTED  TO  THE   FACULTY 

OF  THE  OGDEN   GRADUATE   SCHOOL  OF  SCIENCE 

IN  CANDIDACY   FOR  THE  DEGREE   OF 

DOCTOR  OF   PHILOSOPHY 

DEPARTMENT   OF   PHYSICS 


BY 

FABIAN  MILLER  KANNENSTINE 


Private  Edition,  Distributed  By 

THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 

CHICAGO,  ILLINOIS 


Reprinted  from 
THE  ASTROPHYSICAL  JOURNAL,  Vol.  LV,  No.  4,  May  1922 


AlKfiO"; 


EXCHANGE 


FORMATION  AND  LIFE  OF  METASTABLE  HELIUM 
BY  FABIAN  M.  KANNENSTINE 

ABSTRACT 

Current- potential  curves  for  alternating  and  intermittent  arcs  in  pure  helium  between 
a  Wehnelt  cathode  and  nickel  anode  were  obtained  by  use  of  a  Braun-tube  oscillograph, 
for  pressures  of  from  0.06  to  2  mm.  With  alternating  voltages,  the  striking  potential 
for  each  half-cycle  was  29  volts  or  more  for  frequencies  of  200  or  less,  but  for  frequencies 
of  220  cycles  and  above,  the  current  started  at  about  5  volts  and  soon  reached  a 
saturation  value  until  about  25  volts  caused  a  further  increase.  In  all  cases  the 
breaking  potential  was  about  5  volts.  To  form  the  intermittent  arc,  the  voltage  was 
periodically  dropped  from  36  to  some  lower  predetermined  value.  If  this  lower 
potential  was  less  than  4.6  volts,  the  arc  was  suddenly  extinguished;  if  between  4.6 
and  23.7  volts,  the  arc  died  out  gradually;  and  if  it  was  25.3  volts  or  above,  the  arc 
was  maintained.  All  the  foregoing  values  include  a  correction  of  i.i  volts  added  to 
the  actual  readings  to  bring  them  in  agreement  with  accepted  results. 

Evidence  as  to  the  life  of  metastable  helium. — To  explain  various  experimental 
results,  it  has  been  suggested  that  atoms  of  helium,  partially  ionized  by  electronic 
collisions  of  20.8  volts  or  more  and  hence  capable  of  complete  ionization  by  further 
impacts  of  4.8  volts  or  more,  remain  in  this  metastable  condition  for  an  appreciable 
time.  The  foregoing  results  not  only  confirm  this  suggestion  but  also  show  that 
under  the  circumstances  of  this  experiment  the  metastable  form  persists  for  about 
0.0024  second. 

INTRODUCTION 

Experiments  with  electron  impacts  in  helium  have  led  to  a 
value  for  the  minimum  ionizing  potential  of  25.3  volts  and  to  two 
radiating  potentials,  one  at  20.5  volts1  and  another  at  21.3  volts.2 
Franck  and  Reiche3  and  Franck  and  Knipping2  conclude  that  the 
partially  ionized  state  formed  when  the  atom  has  been  excited  by 
the  impact  of  20.5  volt  electrons  is  a  "metastable"  state,  which  may 
persist  for  some  time  and  may  be  even  permanently  stable  in  abso- 
lutely pure  helium.  This  conclusion  is  drawn  from  the  fact  that 
the  radiation  usually  observed  with  electron  impacts  at  this  voltage 
disappeared  when  the  helium  was  very  pure,  indicating  that  the 
displaced  electrons  do  not  return  to  the  normal  state.  In  slightly 
impure  helium  they  also  observed  that  the  radiation,  as  measured  by 

1  F.  Horton  and  A.  C.  Davies,  Philosophical  Magazine,  42,  746,  1921;  J.  Franck 
and  P.  Knipping,  Physikalische  Zeitschrift,  20,  481, 1919;  K.  T.  Compton,  Philosophical 
Magazine,  40,  553,  1920;  F.  S.  Goucher,  Proceedings  Physical  Society  of  London,  33, 
13,  1920. 

2J.  Franck  and  P.  Knipping,  loc.  cit.  Zeitschrift  fur  Physik,  i,  320,  1920. 
F.  Horton  and  A.  C.  Davies,  loc.  cit. 

3  Zeitschrift  fur  Physik,  i,  54,  1920. 

345 


346  FABIAN  M.  KANNENSTINE 

its  photo-electric  effect,  appeared  to  continue  for  a  brief  time  after 
the  exciting  electron  impacts  ceased. 

Experiments  by  Paschen,1  which  show  that  in  pure  excited  helium 
resonance  radiation  may  be  caused  by  the  infra-red  line  10830  A, 
also  support  the  conception  of  a  metastable  state.  All  the  energy 
of  this  wave-length  absorbed  by  the  excited  atoms  was  found  to 
be  re-emitted  as  radiation  of  this  same  wave-length.  In  terms  of 
Bohr's  model  this  means  that  the  radiation  is  absorbed  by  atoms 
which  have  already  absorbed  the  energy  corresponding  to  the 
radiating  potential  20.5  volts,  and  that  the  displaced  electrons 
return  to  this  partially  ionized  state  only,  and  not  to  the  normal 
state. 

The  semi-stable  existence  of  partially  ionized  helium  atoms  is 
also  useful  in  explaining  the  phenomena  in  low-voltage  arcs  in 
helium,  especially  the  striking  of  the  arc  at  20.5  volts  with  dense 
electron  currents.2  The  phenomena  of  successive  impacts  as  dis- 
cussed by  Compton3  are  of  much  more  importance  when  the  par- 
tially ionized  atoms  have  a  relatively  long  life.  The  metastable 
helium  should  be  completely  ionized  by  electrons  with  a  speed 
corresponding  to  a  potential  of  25.3  —  20.5  =4.8  volts.  If  we  assume 
that  the  metastable  state  may  be  produced  by  the  arc  radiations,4 
then  an  arc  might  be  maintained  with  dense  enough  electron 
currents  down  to  4.8  volts.  It  is  of  interest  that  in  this  way  Comp- 
ton, Olmstead,  and  Lillie  have  maintained  an  arc  with  potentials 
as  low  as  8  volts  between  the  electrodes.  Horton  and  Davies5 
report  an  unsuccessful  attempt  to  observe  the  ionization  of 
abnormal  helium  atoms  by  bombardment  with  electrons  of  4.8 
volts. 

In  this  paper  experiments  with  helium  arcs  are  described  which 
show  that  helium  may  exist  temporarily  in  a  metastable  form,  and 
that  arcs  may  be  maintained  in  it  with  3.5  volts.  An  approximate 
determination  of  its  life  is  also  reported. 

1  Annalen  der  Physik,  45,  625,  1914. 

2  K.  T.  Compton,  E.  G.  Lillie,  P.  S.  Olmstead,  Physical  Review,  16,  282,  1920. 

3  Physical  Review,  15,  476,  1920. 

« O.  W.  Richardson  and  C.  B.  Bazzoni,  Nature  98,  5,  1916;  F.  Horton  and  A.  C. 
Davies,  op.  cit.,  42,  752,  1921. 
5  Ibid.,  42,  763,  1921. 


METASTABLE  HELIUM  347 

ARCS   WITH   LOW-FREQUENCY   VOLTAGES 

The  arcs  were  formed  between  a  Wehnelt  cathode  and  nickel 
anode  in  a  pyrex  tube  containing  carefully  purified  helium.  The 
helium  was  purified  by  passing  over  heated  copper  oxide  and 
charcoal  immersed  in  liquid  air.  Before  passing  into  the  experi- 
mental tube  the  helium  passed  through  two  additional  charcoal 
tubes  and  a  liquid-air  trap.  The  tube  and  liquid-air  trap  were  baked 
out  at  450°  C.  for  several  days  before  the  helium  was  admitted. 

In  the  first  experiments  the  helium  was  excited  by  an  alternat- 
ing e.m.f.  An  electromagnetic  oscillograph  was  used  to  obtain 
curves  showing  the  variation  of  current  and  voltage  with  time. 
From  such  curves  the  current-voltage  curve  may  be  obtained. 

A  decided  dissymmetry  in  the  growth  and  decay  of  the  arc 
current  could  be  observed,  and  also  an  apparent  continuation  of 
the  arc  current  down  to  very  low  voltages.  To  obtain  greater 
sensitiveness  and  to  avoid  the  possible  distortion  from  the  inertia 
of  the  oscillograph  elements,  a  sensitive  Braun  tube  oscillograph 
was  used  in  most  of  the  later  experiments.  The  electrostatic 
deflecting  plates  of  the  Braun  tube  were  connected  across  the  anode 
and  cathode  of  the  helium  tube,  while  the  arc  current  passed  through 
the  electromagnetic  deflecting  coils.  The  electrostatic  and  electro- 
magnetic deflections  were  at  right  angles,  so  that  the  figure  on  the 
screen  of  the  Braun  tube  was  a  graph  in  rectangular  co-ordinates 
of  the  impressed  e.m.f.  and  the  arc  current.  The  deflecting  coils 
were  so  adjusted  that  the  thermionic  current  alone  did  not  produce 
any  observable  deflection. 

Figure  i  shows  the  kind  of  curve  obtained  with  a  6o-cycle 
e.m.f.  in  helium  at  a  pressure  of  2  mm.  It  is  seen  that  at  a  certain 
voltage  the  arc  strikes,  and  as  the  voltage  increases  the  current 
increases  also.  When  the  voltage  decreases  the  current  decreases 
at  the  same  time,  but  continues  with  considerable  magnitude  until 
the  voltage  reaches  a  value  of  about  4  volts,  when  the  current 
ceases  to  flow.  Careful  observation  showed  a  sudden  drop  to 
zero  at  this  lower  voltage.  This  continuation  of  the  arc  to  very 
low  potentials  may  be  attributed  to  the  persistence  of  a  metastable 
form  of  helium  in  which  an  arc  may  be  maintained  down  to  about 
4  volts.  Two  other  breaks  in  the  down  curve  may  be  observed 


348 


FABIAN  M.  KANNENSTINE 


just  below   the   striking  potential   of   the  helium   itself.     These 
breaks  will  be  referred  to  later. 

This  type  of  curve  was  observed  with  60  cycles  at  pressures 
down  to  0.06  mm.  Down  to  i  mm  pressure  the  striking  potential 
was  about  28  volts,  and  at  the  dying-out  potential  of  about  4  volts 
a  very  decided  drop  of  the  arc  current  to  zero  was  observed.  As 
the  pressure  was  decreased  below  i  mm  the  striking  potential 

t* 


FIG.  i 


FIG.  2 


increased  and  the  dying-out  potential  became  less  certain,  i.e., 
the  drop  to  zero  current  was  not  so  sudden  as  at  higher  pressures. 
At  0.06  mm  the  striking  potential  had  increased  to  about  40  volts 
and  the  dying-out  potential  to  about  20  volts.  Figure  2  shows  the 
type  of  curve  obtained  at  0.06  mm  pressure.  At  all  these  pressures 
the  filament  temperature  was  approximately  constant.  The  mean 
arc  current  varied  from  25  milliamperes  at  2  mm  pressure  to  10 
milliamperes  at  0.06  mm  pressure. 

If  any  impurities  are  present  one  would  expect  their  effect  to 
be  more  marked  at  lower  pressures.  Franck  and  Knipping  have 
assumed  that  me ta stable  helium  will  not  return  to  normal  helium 
except  in  the  presence  of  impurities,  so  that  with  larger  amounts 
of  impurities  we  might  expect  the  metastable  helium  to  be 
destroyed  in  a  shorter  time.  This  is  probably  the  interpretation 
of  the  gradual  dying  out  of  the  arc  before  the  voltage  had  dropped 
to  4  volts,  as  was  observed  when  the  pressure  was  very  low. 
Experiments  were  made  directly  with  the  method  discussed  in  the 
next  section  to  see  if  the  life  was  affected  by  impurities  obtained 
by  removing  the  liquid  air  from  the  charcoal  for  a  short  time  till 
the  helium  had  become  very  impure.  The  experiments  indicated 


MET  AST  ABLE  HELIUM  349 

that  while  the  impurities  suppressed  the  amount  of  the  metas table 
helium,  the  part  remaining  had  the  same  life  as  that  formed  in  the 
pure  helium.  The  increase  in  the  striking  potential  as  the  pressure 
was  reduced  is  more  difficult  to  interpret  and  is  probably  connected 
with  the  mechanism  of  the  arc  formation. 

ARCS   WITH  HIGH-FREQUENCY  VOLTAGES 

With  a  sufficiently  high  frequency  the  e.m.f.  should  pass  from 
+4  volts  to  its  negative  maximum  and  back  to  +4  volts  before  the 
metastable  helium  has  disappeared,  and  upon  reaching  4  volts 
the  arc  should  strike,  due  to  the  ionization  of  the  metastable  helium 
still  present. 

Accordingly  various  frequencies  from  60  to  10,000  cycles  were 
impressed  on  the  arc.  An  audion  oscillator  was  used  as  a  source 
for  these  frequencies.  The  author  wishes  to  acknowledge  his 
indebtedness  to  Dr.  J.  P.  Minton  and  Mr.  V.  O.  Knudsen  for  the 
use  of  their  oscillators  in  this  work.  The  alternating  e.m.f. 
impressed  on  the  arc  was  obtained  from  the  secondary  of  an  iron 
core  transformer  having  its  primary  winding  in  the  plate  circuit 
of  the  oscillator. 

Figure  3  shows  the  type  of  figure  observed  above  220  cycles 
per  second  at  2  mm  pressure.  The  arc  struck  at  about  4  volts, 
and  the  current  rapidly  attained 
a  saturation  value  at  which  the 
arc  current  continued  until  the 
voltage  had  reached  about  25  volts, 
when  a  second  sudden  increase  was 
obtained.  The  decreasing  part  of 
the  curve  was  above  the  increasing 
part  but  fell  to  zero  at  the  lower 
striking  voltage. 

With  200  cycles  no  appreciable  FlG  3 

arc  formation  at  the  lower  voltage 

could  be  observed,  indicating  that  the  metastable  helium  practi- 
cally disappeared  in  less  than  approximately  Tfo  of  a  second.  The 
striking  of  the  arc  at  about  4  volts  with  220  cycles  indicates 
that  enough  of  the  metastable  helium  was  left  after  approximately 
of  a  second  to  give  an  observable  indication  of  ionization. 


e. 


350 


FABIAN  M.  KANNENSTINE 


All  the  figures  above  220  cycles  were  similar,  except  that  as 
the  frequency  was  increased  the  striking  at  about  4  volts  became 
more  and  more  marked. 

In  using  the  oscillator  as  a  source  of  alternating  e.m.f.  smooth 
curves  were  not  always  obtained.  As  the  mean  arc  current  was 
increased,  by  raising  the  electron  emission,  certain  parts  of  the 
curve  became  wavy.  This  was  particularly  noticeable  on  the 
striking  of  the  arc  at  about  25  volts.  Instead  of  the  arc  current 
increasing  as  in  Figure  i,  it  increased  in  a  wavy  form,  indicating 
that  the  impressed  voltage  rose  and  fell  during  this  striking  period. 
This  is  provisionally  attributed  to  self-induction  in  parts  of  the 
circuit  or  to  the  presence  of  iron  in  the  oscillating  circuit. 

ARCS  WITH  INTERMITTENT  CONSTANT  VOLTAGES 

In  another  series  of  experiments  intermittent  constant  voltages 
from  a  battery  were  impressed  between  the  hot  cathode,  C,  and 
the  anode,  A  (see  Fig.  4).  When  the  mercury  interrupter,  H,  was 

open  the  voltage  impressed 
on  the  electrodes  was  about 
35  volts,  and  when  the  in- 
terrupter shorted  the  section 
BD,  the  voltage  dropped  to 
a  value  dependent  upon  the 
position  of  the  slider  B.  In 
this  way  it  was  possible  to 
impress  a  voltage  of  about 
35  volts  and  suddenly  change 
this  voltage  to  a  lower  value. 
The  arc  current  was  passed 
through  the  deflecting  coils, 
M,  of  the  Braun  tube.  The  spot  was  examined  directly  and  also 
with  a  rotating  mirror.  Since  it  was  rather  difficult  to  observe 
the  spot  in  the  rotating  mirror  the  following  arrangement  was  also 
used.  A  uniform  time  deflection  was  obtained  by  connecting  the 
deflecting  plates  to  a  potentiometer  with  a  rotating  contact,  which 
was  mounted  on  the  shaft  of  the  mercury  interrupter.  The  electro- 
static deflection  was  therefore  proportional  to  the  time  and  the 


FIG.  4 


METASTABLE  HELIUM 


351 


magnetic  deflection  proportional  to  the  current  through  the  arc. 
These  two  deflections  were  at  right  angles.  Furthermore,  since 
the  rotating  contact  was  mounted  on  the  interrupter  shaft,  the 
figure  produced  was  stationary  and  could  be  carefully  examined 
as  the  slider  B  was  moved. 

With  the  lower  voltage  below  3.5  volts  and  without  the  time 
deflection  two  spots  appeared  on  the  Braun  tube  screen,  the  upper 
one  corresponding  to  the  current  for  35-volt  excitation  and  the 
lower  one  for  zero  current.  Using  the  time  deflection,  it  was  seen 
that  the  arc  was  extinguished  in  a  time  too  short  to  be  so  observed, 
as  shown  at  the  right  of  Figure  5.  Only  a  very  faint  vertical  line 
appeared,  since  the  spot  moved  very  rapidly  from  the  upper  to  the 
lower  positions. 

As  the  lower  voltage  was  increased  to  3.5  volts  the  lower  line 
suddenly  became  curved  as  shown  in  Figure  6,  indicating  a  gradual 

u  u 


FIG.  5 


FIG.  6 


decrease  of  the  current  to  zero  value.  This  sudden  curving  of  the 
lower  line  at  3.5  volts  may  be  attributed  to  the  presence  of  metas- 
table  helium  produced  during  the  35-volt  excitation.  That  is,  the 
voltage  dropped  from  35  volts  to  3.5  volts  in  a  time  short  compared 
with  the  life  of  the  metastable  helium,  and  the  arc  was  maintained 
by  3.5  volts  as  long  as  the  metastable  helium  was  present. 

The  dying  out  of  the  arc  continued  to  have  the  same  appearance 
until  the  lower  voltage  had  been  increased  to  22.6  volts,  when  a 
third  spot  appeared  slightly  above  that  for  zero  current  as  in  Fig- 
ure 7.  Using  the  time  deflection,  it  was  seen  that  the  arc  was 
maintained  for  a  brief  time  with  this  voltage  before  dying  out. 

A  slight  further  increase  of  the  lower  voltage  to  23.6  volts 
brought  out  still  another  spot,  as  in  Figure  8,  and  the  time  deflec- 
tion showed  a  dying  out  of  the  current  in  two  stages  as  shown. 
The  voltages  for  these  two  breaks  in  the  decrease  correspond  to 


352 


FABIAN  M.  KANNENSTINE 


the  voltages  for  the  two  breaks  observed  in  the  down  curve  of  the 
alternating  e.m.f.  figure  already  described.  This  decrease  in  two 
stages  may  be  connected  with  the  fact  of  the  existence  of  two  radiat- 
ing potentials  of  helium  0.8  volts  apart  corresponding  to  the  first 
coplanor  orbit  (helium)  and  the  first  crossed  orbit  (parhelium)  on 
Franck  and  Knipping's  interpretation. 

With  a  slightly  increased  lower  voltage  of  24.2  volts  the  arc 
was  maintained.  The  two  lower  spots  of  Figure  8  disappeared, 
and  the  arc  current  jumped  between  the  two  values  indicated  by 
the  two  upper  spots. 


FIG.  7 


FIG.  8 


These  phenomena  were  qualitatively  the  same  at  pressures 
ranging  from  2  mm  down  to  0.06  mm.  The  above  exact  determina- 
tions were  all  made  visually  with  the  Braun  tube  oscillograph,  but 
for  the  purpose  of  illustration  photographs  made  with  the  electro- 
magnetic oscillograph  show  qualitatively  the  same  phenomena, 
although  with  less  sharpness  in  the  details.  Plate  VI  shows  the 
various  changes  which  occur  as  the  lower  voltage  is  increased  from 
zero  to  24.2  volts.  These  oscillograms  were  obtained  by  passing 
the  arc  current  through  an  element  of  the  electromagnetic  oscillo- 
graph and  photographing  the  curve  while  the  mercury  interrupter 
was  operating.  In  a  the  lower  voltage  was  below  3.5  volts,  and 
it  is  seen  that  the  arc  current  drops  to  zero  in  a  very  short  time. 
The  curving  of  the  lower  current  line  when  the  lower  voltage  reaches 
3.5  volts  is  shown  in  b.  The  brief  maintenance  of  the  arc  at  a  lower 
voltage  of  22.6  volts  is  shown  in  c,  and  the  decrease  in  two  stages 
at  a  lower  voltage  of  23.6  is  shown  in  d.  The  maintenance  of  the 
arc  at  24.2  volts  is  shown  in  e. 

A  number  of  readings  were  taken  of  these  voltages,  and  then 
the  cathode  current  was  reversed  in  order  to  eliminate  the  drop  in 
voltage  along  the  cathode,  and  another  set  of  readings  taken.  The 


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